ECG is a recording of the electrical activity of the heart.
Six electrodes are placed across the chest and one on each arm and leg.
The ECG machine receives and amplifies the signals to produce and record an electrical trace.
A trained assistant practitioner performs the procedure, which takes about ten minutes.
The results are passed on to the doctor or specialist nurse to help them detect problems with heart rate or rhythm.
British Heart Foundation: Your guide to ECG (electrocardiogram), heart disease test
Please see also the British Heart Foundation website: ECG.